Method for the automatic control of the quality of the bottom and top product in a continuous distillation process



Feb. 18, 1969 J. E'. RIJNSDORP ET AL METHOD FOR THE AUTOMATIC CONTROL OFTHE QUALITY OF THE BOTTOM AND TOP PRODUCT IN A CONTINUOUS DISTILLATIONPROCESS Filed Nov. 26, 1965 CONTROLLER ACCUIULATQR JOHANNES E. RIJNSDORPJOHANNES A. VAN KAMPEN HANS BOLLEN THEIR ATTORNEY Feb. 1 1969 J. E.RIJNSDORP E AL METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THEBOTTOM 7 AND TO? PRODUCT IN A CONTINUOUS DISTILLATION PROCESS Filed NOV.26, 1965 Sheet wmwODQwZSE. awk

En: 25 5: I! $3228 INVENTORS:

JOHANNES E. RIJNSDORP JOHANNES A. VAN KAMPEN HANS BOLLEN BY: THEIRATTORNEY Feb. 18, 1969 J. E. mmsoon ETAL 3,4 27

METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM AND TOPPRODUCT IN A CONTINUOUS DISTILLATION PROCESS INVENTORS:

JOHANNES E. RIJNSDORP JOHANNES A. VAN KAMPEN HANS BOLLEN THEIR ATTORNEFeb. 18, 1969 J. E. RIJNSDORP ET AL 3,423,527 METHOD FOR THE AUTOMATICCONTROL OF THE QUALITY OF THE BOTTOM AND TOP PRODUCT IN A CONTINUOUSDISTILLATION PROCESS Filed Nov. 26, 1965 Sheet WITT 57% M a .3 a

W e W L J Ml lllllll IIIAIILT 5:258 T; 5:15 i a 2L lNVENTORS:

JOHANNES E. RIJNSDORP THEIR ATTORNE;

Feb. 18, 1969 J. E. RIJNSDORP L METHOD FOR THE AUTOMATIC CONTROL OF THEQUALITY OF THE BOTTOM AND TOP PRODUCT IN A CONTINUOUS DISTILLATIONPROCESS Filed Nov. 26, 1965 CONTROLLER 9 3 FJI.

Sheet 5 of '7 FIG.

CONTROLLER Q n CONTROLLER INVENTORS:

JOHANNES E. RIJNSDORP JOHANNES A. VAN KAMPEN HANS BOLLEN B WW 7/ THElRATTORNE Feb. 18, 1969 J. E. RIJNSDORP ETAL 3, 8,5

METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM AND TOPPRODUCT IN A CONTINUOUS DISTILLATION PROCESS FIG. 6

INVENTORS JOHANNES E. RIJNSDORP JOHANNES A. VAN KAMPEN HANS BOLLEN THEIRATTORNE Feb. 18, 1969 J. E. RIJNSDCRP ETAL 3,428,527 METHOD FOR THEAUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM AND TOP PRODUCT IN ACONTINUOUS DISTILLATION PROCESS Filed NOV. 26, 1965 Sheet m a m.

ECEEEE L 5252. Nm

lll rl l 5:228 I I l L 2 INVENTOR$= JOHANNES E, RIJNSDORP JOHANNES A.VAN KAMPEN HANS BOLLEN THEIR ATTORNEY United States Patent 3,428,527METHOD FOR THE AUTOMATIC CONTROL OF THE QUALITY OF THE BOTTOM AND TOPPRODUCT IN A CONTINUOUS DISTILLATION PROCESS Johannes E. Rijnsdorp,Johannes A. van Kampen, and Hans Bollen, Amsterdam, Netherlands,assignors to Shell Oil Company, New York, N.Y., a corporation ofDelaware Filed Nov. 26, 1965, Ser. No. 510,021 Claims priority,application Netherlands, Nov. 30, 1964, 6413882 US. Cl. 203-1 5 ClaimsInt. Cl. B01d 3/42, 3/14 ABSTRACT OF THE DISCLOSURE Automaticallycontrolling a continuous distillation process for the separation of feedstream into a top and a bottom product stream in a column using refluxand reevaporation. Upon a control action for keeping the quality of thebottom product constant the quality of the top product does not change.This is achieved by analyzing the quality of the bottom product andadjusting the feed flow rate to maintain the quality thereof andadjusting some of the remaining process variables to maintain the liquidlevels in the accumulator and in the bottom of the column betweendesired limits and the ratio between the vapor flow and the liquid flowin the rectifying section of the column at a constant value.

The invention relates to a method and apparatus for automaticallycontrolling a continuous distillation process for the separation of anintake mixture into a top and a bottom product stream in a column usingreflux and reevaporation whereby upon a control action for keeping thequality of the bottom product constant, the quality of the top productdoes not change or changes only very slowly.

If often occurs in technical practice that both the top product and thebottom product withdrawn from a continuous distillation process have tomeet specific requirements as regards quality. For example, it may berequired that the top product contain no more than a specified smallamount of impurity, while the bottom product should as far as possiblebe free from the main constituent of the top product. An example of sucha situation in the oil industry is the removal of a valuable lightproduct, such as isopentane, from a mixture of hydrocarbons where thepurity of the isopentane must be at least 97% and where the bottomproduct may contain no more than 2% isopentane.

In such a distillation process, it may also be of importance to weightthe quality of the bottom product highest, for instance, if that producthas to be supplied against specification. Accordingly, continuous directcontrol of the quality of the bottom product must be provided tomaintain the bottom product at its desired value. However, at the sametime it is also desirable to permit the quality of the top product tovary as little as possible.

In order to reach this aim of maintaining the quality of the bottomproduct on specification while permitting the top product quality tovary very little, it is attempted in controlling processes of this typeto prevent dynamic coupling between the stripping and the rectifyingsections of the column. That is to say, when adjusting the processvariables to a specific ratio between the vapor flow and the liquid flowin the stripping section of the column in order to maintain the qualityof the bottom product, the ratio between the vapor flow and the liquidflow in the rectifying section of the column should be disturbed as ICClittle as possible. In this way it is ensured that no or substantiallyno deviation occurs in the quality of the top product as a result ofthat control action in the stripping section of the column.

J. K. Kennedy in his thesis, The Control of Distillation Columns,Manchester, 1963, on pages 173 and 192, describes a control scheme bywhich it is ensured that to a certain extent decoupling of the strippingsection from the rectifying section of the column occurs upon a controlaction for maintaining the quality of the bottom product. According tothis scheme the degree of reevaporation is adjusted by a quality meterfor the product on one of the trays in the column while the reflux ratiois kept at a desired value by means of a ratio controller. The intendeddecoupling is here obtained due to the fact that because of the ratiocontrol at the top of the column the reflux flow follows any change inthe vapor flow over the top of the column, and in such a way that theratio between those flows remains constant. Accordingly, the quality ofthe top product also ordinarily remains substantially constant. However,if there are disturbances that require a permanent change of the processconditions, for example, a different composition of the feed, then theset value of the ratio controller has to be changed. Such a change canbe carried out automatically, for example, with the aid of a signalderived from a meter or analyzer for measuring the quality of the topproduct.

With the mode of control described by Kennedy the feed stream can beadjusted to an independent value. It is not possible, however, with thiscontrol method to adjust the top product flow, the reflux flow, thebottom product flow or the heat flow to the reboiler to an independentlycontrolled value.

The process according to the invention, which is likewise aimed atpreventing dynamic coupling between the two column sections as far aspossible, provides for the possibility of making a choice from the fourlast-mentioned flows for adjusting one of those flows at an independentvalue, as a result of which the variety of the applications isincreased. In addition, compared with the mode of control described byKennedy, an equivalent or improved decoupling between the column sectionis obtained, depending on the variant chosen.

Briefly, the characteristics of the process according to the inventionare that:

(a) the quality of the bottom product is controlled by a controllerwhich adjusts the feed stream flow, and

(b) the degree of reevaporation, the degree of preheating the feed, thetop product flow, the bottom product flow, the degree of reflux and/orthe degree of cooling in the condenser are adjusted in such a way thatthe liquid levels in the accumulator and in the bottom vary betweendesired limits and that the ratio between the vapor flow and the liquidflow in the rectifying section of the column is as far as possiblemaintained at a desired predetermined value.

If the quality of the bottom product does not meet the requirement orrequirements made, then the feed stream is increased or decreased as theneed may be until the ratio between the vapor flow and the liquid flowin the stripping section of the column that is necessary for the desiredquality is reached. The correcting action therefore concerns the liquidflow in the stripping section of the column and since this flow has noinfluence on the ratio between the vapor flow and the liquid flow in thestripping section of the column that is necessary for the desiredquality is reached. The correcting action therefore concerns the liquidflow in the stripping section of the column and since this flow has noinfluence on the ratio between the vapor flow and the liquid flow in therectifying section of the column, it follows that the desired decouplingbetween the two column sections has been obtained.

According to the invention this mode of control is combined with controlsystems which in themselves are known, for those process conditionswhich must be controlled.

It is possible by an appropriate choice of those combinations to satisfyvarious wishes as regards process conditions. This flexibility withinthe range of the invention is likewise the result of the fact that thecontrol action for maintaining the quality of the bottom productprimarily concerns the liquid fiow in the stripping section of thecolumn.

If the temperature of the feed stream when entering the column is higherthan the boiling temperature on the feed tray, then the vapor flow willbe larger in the rectifying section of the column than in the strippingsection. If the temperature of the feed stream is lower, then that vaporflow will be smaller. In both cases the size of the vapor flow in therectifying section of the column will change upon a change of the feedstream. Accordingly, as the reflux ratio is higher, the effect on theratio between the liquid flow and the vapor flow in the rectifyingsection becomes smaller, that effect even becoming negligibly small.However, it is possible within the range of the invention totake accountof this secondary effect in such a way that the occurring change of thesize of the vapor flow in the rectifying section is rapidly compensated.

The invention will be more clearly understood from the followingdetailed description of the invention when taken in conjunction with theaccompanying drawings where- FIGURE 1 is a diagrammatic representationof a control system according to the invention for a distillationapparatus wherein the quantity of heat supplied to the distillationcolumn is maintained at an independently controlled value;

FIGURE 2 shows an alternate embodiment of the control scheme illustratedin FIGURE 1;

FIGURE 3 is a diagrammatic representation of a control system accordingto the invention for a distillation apparatus wherein the top productstream is maintained at an independently controlled value;

FIGURE 4 shows an alternate embodiment of the con trol schemeillustrated in FIGURE 3;

FIGURE 5 is a diagrammatic representation of a control system accordingto the invention for a distillation process wherein the reflux flow ismaintained at an independently controlled value;

FIGURE 6 is a diagrammatic representation of a control system accordingto the invention for a distillation apparatus wherein the bottom productflow is maintained at an independently controlled value; and

FIGURE 7 shows a modification of the control scheme illustrated inFIGURE 3 which is of particular advantage when the feed is introduced tothe column at a temperature below the boiling temperature.

The apparatus for carrying out the distillation process control schemementioned above is illustrated in FIGURE 1 wherein there is shown adistillation column 1 of any suitable design which contains vapor-liquidcontacting elements, such as a vertical series of bubble or valve trays.The feed is continually introduced in either a liquid or vapor form intothe column 1 to an intermediate stage or feed tray through a pipeline 2containing a preheater 3 if such is required; the top product in thevapor phase is discharged through a pipeline 4; and the liquid bottomproduct is discharged through a pipeline 5.

Connected to the top product column discharge line 4 is a condenser 6which condenses the vapor in the line 4; the condensate then flows intoa top product accumulator 7. The condensate collected in the accumulatoris discharged through a pipeline 8 and a portion of the condensate isreturned to the top of the column 1 for refiux by a pipeline 9, whilethe remainder of the discharged condensate is passed through a dischargepipeline 10 as the liquid top product output stream from thedistillation process.

Reevaporation for the column 1 is eifected by passing a stream of liquidfrom the bottom of the column via a pipeline 11 to a reboiler 12 whichmay, for example, be a heat exchanger, and returning the heated stream,either entirely or partially in the vapor phase to the column l via apipeline 13. Heat is supplied to the reboiler 12 via a pipeline 14. Theparticular manner in which reevaporation is attained, however, forms perse no part of this invention since reevaporation may be accomplished inother ways, e.g., with a heating coil in the bottom of the column.

Turning now to the control of the distillation process shown in FIGURE1, as indicated above, the control schemes according to the inventionare for processes wherein the feed flow rate is adjusted to maintainquality of the bottom product at a desired value. Accordingly, the feedfiow line 2 is provided with a control valve 15, the position of whichis controlled or adjusted by means of a controller 16. In order todetermine the quality of the bottom product, quality measuringinstrument or analyzer 17, e.g., a process chro-matograph, is providedwhich continuously samples the bottom product flowing in the flow line 5and produces an output signal corresponding to the measured quality tothe the normal input of the controller 16 which compares the measuredvalue of the quality of the bottom product with a signal related to thedesired value of the bottom product quality, i.e., the set value of thecontroller 16. The set value input signal of controller 16, and of allother controllers or adjustable control elements shown in the drawingswhich do not have set values which are adjusted by a process variable,is indicated by means of an arrow, as indicated by the reference numeral18. Any differences resulting from the comparison by the controller 16will cause the output signal from the controller 16 to adjust the valve15 in a manner to decrease the difference and hence adjust the feed flowrate for the distillation process to the value which will produce abottom product of the desired quality may be maintained.

The bottom liquid product stream flow rate is adjusted by means of acontrollable valve 19' connected in the pipe line 5. The valve 19 iscontrolled by means of a controller 20 (which compares the measuredvalue of the liquid level in the bottom of the column, as determined bythe liquid level gauge 21, with a signal proportional to the desiredlevel in the bottom of the column, i.e., the set value of thecontroller. The controller 20' attempts to keep the liquid level in thebottom of the column within desired limits and hence any differencesresulting from the comparison will produce an output signal from thecontroller 20 which tends to adjust the valve 19 in such a way that thedifference is decreased. Thus, if at a given moment the measured valueof the liquid level is larger than the set or desired value, the signalsupplied from the controller 20 tends to open the valve 19 wider.

As also indicated above, according to this embodiment of the invention,the heat flow to the column via the reboiler is maintained at anindependently controlled value. The control of the heat flow to thereboiler 12 is provided by a controller 22 whose output is connected toa controllable valve 23 in the flow line 14. The normal input ofcontroller 23 is connected to the output of a meter 24 whichcontinuously measures the quantity of heat supplied to the columnthrough the reboiler 12. The meter 24 performs this measurement, in amanner well known in the art, by combining the output signals from afiowmeter 25 in the line 14 and a pair of transducers 26 and 27 whichmeasure the temperature of the heating medium before and after passingthrough the reboiler 12. It should be noted that if the heating mediumpassing through flow line 14 has a constant enthalpy, for example, oilof a constant temperature or steam of a constant temperature and pressure, then only the signal from flowmeter 25 need be used to obtain asufiiciently accurate indication of the heat flow to the reboiler 12.The controller 22 compares the output signal from. heat fiowmeter 24with the desired value of the heat flow, i.e., the set value ofcontroller 22, and adjusts valve 23 to maintain the heat flow at thedesired value.

By means of the control actions just described, the quality of thebottom product is maintained at the desired value. v'However, tomaintain the quality of the top product, the ratio between the liquidand vapor flows in the rectifying or upper section of the column 1 mustbe maintained at a constant value by controlling the ratio between thereflux and top product flows. According to this embodiment of theinvention, the reflux flow is adjusted by means of a controller 30 and acontrollable valve 31 connected in the reflux flow line 9. Thecontroller 30 compares the output signal from a liquid level gauge 3-2,which measures the liquid level in the accumulator 7, with the desiredvalue of the liquid level in the accumulator and adjusts the valve 31 tomaintain the liquid level in the accumulator between desired limits. Thetop product flow is controlled by a ratio controller 33 whose output isconnected to a control valve 34 in the top product flow line 10. The twoinputs of the ratio controller are provided by a pair of flowmeters 35and 36 which measure the reflux flow and the top product flowrespectively. The ratio controller 33 compares the two input signalsthereto and adjusts the top product flow via control valve 34 tomaintain a predetermined ratio between the reflux and top product flows.With this control scheme any variation in the vapor flow in therectifying section will result in a change in the liquid level in theaccumulator which, via the gauge 32, the controller 30 and the valve 31,causes a corresponding change in reflux flow and via ratio controller 33and valve 34 a proportionate change in the top product flow to restorethe desired vapor-to-liquid flow ratio in the rectifying section of thecolumn and thereby maintain the quality of the top product.

With this control scheme it is not important at what temperature thefeed enters the column. The additional effect, if any, of a change ofthe vapor flow in the rectifying section of the column upon adjustmentof the feed stream caused by the temperature thereof differing from theboiling temperature on the feed tray, is immediately compensated by theratio control at the top of the column.

It should be noted that, although not shown, the set value of controller33 can be derived from a meter and a controller for the quality of thetop product. Moreover, it is also possible according to this controlmode for the outputs of level controller 30 and ratio controller 33 tobe interchanged, i.e., the output of level controller 30 can beconnected to valve 111 when the output of ratio controller 33 isconnected to valve 9.

Referring now to FIGURE 2, there is shown an alternate embodiment orvariant of the control scheme of FIGURE 1. In this and any followingfigures, components previously used are represented by like referencenumerals. According to this variation, the output of flowmeter 35 forthe reflux flow is connected to normal input of a controller 37, whichhas its output connected to the control valve 31, of a controller 37while the output of flow meter 36 for the top product flow is connectedto the normal input of a controller 38 which has its output connected tothe control valve 34. The set value inputs of the controllers 37 and 38are connected to respective outputs of a ratio adjusting device 39 whoseinput in turn is connected to the output of liquid level controller 30.The ratio adjusting device 33, which may, for example, be a pair ofamplifiers having a common input terminal and their respective gainsrelated by a desired predetermined ratio, produces a pair of outputsignals which are both derived from the input signal thereto and whosemagnitudes are related by a predetermined ratio. With this arrangement,upon a change of the liquid level in the accumulator 7 both the refluxflow and the top product flow will change in such a way that the ratiobetween those flows remains constant. In the event that it is requiredto change the predetermined ratio, then a quality controller for the topproduct (not shown) can supply the signal by which the ratio between theoutput signals from ratio adjusting device 39 is adjusted.

It is sometimes also desirable to maintain the top product flow at anindependent value. This type of control may be desirable, for example,if the top product flow constitutes the feed for a following processwhich requires a feed stream flow rate having a specified value. Theprocess according to the invention can provide for this variant in thecase where the feed stream for the distillation process is introducedinto the column at a temperature that is equal or approximately equal tothe boiling temperature, by carrying out the control of the process insuch a way that the control of the quality of the bottom product byadjustment of the feed stream is com.- bined with:

(a) Maintenance of the top product flow at an independently controlledvalue;

(b) Adjustment of the reflux flow and of the bottom product flow in sucha way that the liquid levels in the accumulator and in the bottom of thecolumn vary between desired limits;

(c) Maintenance of the heat flow to the reboiler at a predeterminedvalue.

An example of a control scheme according to the invention for carryingout this variant is represented in FIGURE 3. According to thisembodiment of the invention, the top product flow is independentlycontrolled by means of the fiowmeter 36, controller 38 and valve 34. Thecontroller 38 compares the output signal from flowmeter 36 with thedesired value of the top product flow, i.e., the set value of controller38, and adjusts valve 34 to reduce any diflerence therebetween. The feedflow, the reflux flow, the liquid level in the bottom of the column andthe heat flow to the reboiler are controlled in the same manner as inFIGURE 1. With this control scheme, since the feed stream enters thecolumn at a temperature which is equal or approximately equal to theboiling temperature, the control action in the loop comprising analyzer17, controller 16', valve 15, flow line 2, column 11 and flo'w line 5 inorder to maintain the quality of the bottom product does not affect theratio between the vapor flow and the liquid flow in the rectifyingsection of the column. Moreover, since the vapor flow in the column iskept at a specified value by meter 24, controller 22 and control valve23, the ratio between the liquid and vapor flows in the rectifyingsection of the column remains constant as long as the level in theaccumulator 7 does not change. Of course, any changes in the liquidlevel in accumulator 7 which cause the liquid level to vary out of thedesired limits is compensated (for by adjusting the valve 31 to restorethe desired liquid-vapor ratio in the rectifying section.

It should be noted that with the control scheme of FIGURE 3, slow orpermanent changes necessary for maintenance of the desired quality ofthe top product can be effected by changing the set value of controller22. These changes can be done manually or automatically by means of acontroller for the quality of the top product. Additionally, if therequired change is large, for example, when changing to another intakemixture, then it is likewise advantageous 'for the set value for thecontroller 30 for the liquid level in the accumulator 7 to be derivedfrom a meter measuring the quality of the top product. A control schemeincorporating both of these modifications is illustrated in FIGURE 4'wherein an analyzer 40 for measuring the quality of the top product isprovided. The output signal from analyzer 40* is fed to the normal inputof a quality controller 4 whose output signal is passed to the set valueinputs of controllers 22 and 30. Preferably, as illustrated in FIGURE 4,a multiplier 42 is also provided in order to properly adjust themagnitude of the signal derived from controller 41 and supplied tocontroller 30 according to a desired ratio. With the additional controlprovided, if the vapor flow increases due to valve 23 being fartheropened, then the liquid level in the accumulator 7 will rise, resultingin valve 31 being farther opened, thereby preventing unnecessaryfluctuations in the ratio between the vapor flow and the reflux flow.Obviously, if the vapor flow should decrease, the valves 23 and 31 willbe similarly further closed.

The process according to the invention also provides for a variant bywhich the reflux flow can be adjusted at an independent value. This maybe of importance in relation to column properties, for example, if thecolumn is adjusted to give the maximum production. The characteristic ofthis process is that the control of the quality of the bottom product byadjusting the feed stream is combined with:

(a) Maintenance of the reflux flow at an independent value;

(b) Adjustment of the top product flow in such a way that the liquidlevel in the accumulator varies within desired limits;

(c) Adjustment of the bottom product flow in such a way that the liquidlevel in the bottom varies between desired limits; and

(d) Maintenance of the heat flow to the reboiler at a predeterminedvalue.

An example of this control scheme is given in FIGURE 5. The onlydifference between the scheme of FIGURE 5 and that of FIGURE 3 is thatthe manner of controlling the valves 31 and 34 is interchanged. That isto say, the valve 34 is controlled by level controller 30 While thevalve 31, and hence the reflux flow, is controlled to an independentlyderived value by means of flowmeter 35 and controller 37. It should benoted that with this embodiment, as with that of FIGURE 3, the feedstream should be introduced to the column at a temperature approximatelyequal to the boiling temperature.

The process according to the invention further provides for a variant bywhich the bottom product flow can be adjusted at an independent value.This control mode may be of importance, for example, if the bottomproduct stream constitutes the feed for another process, which requiresa feed stream flow rate having a specified value. This mode of controlaccording to the invention is achieved by combining the control of thequality of the bottom product through adjustment of the feed streamwith:

(a) Maintenance of the bottom product flow at an independentlycontrolled value;

(b) Adjustment of the top product flow in such a way that the liquidlevel in the accumulator is maintained with desired limits;

(c) Adjustment of the reflux flow in such a way that the ratio betweenthe reflux flow and the top product How is maintained at a desiredvalue;

(d) Adjustment of the heat flow to the reboiler by means of a signalderived from a gauge measuring the liquid level in the bottom of thecolumn and to increase heat flow to the reboiler when the liquid levelin the bottom of the column rises and decrease heat flow when the columnbottom liquid level drops.

FIGURE 6 represents a scheme for this variant of a basic control scheme.According to this embodiment of the invention, the reflux flow, the topproduct flow and the feed flow are controlled in the same manner asdescribed with respect to FIGURE 1. The bottom product flow in flow line5, however, is controlled by means of a controller 43, whose normalinput is connected to a flowmeter 44 in the flow line 5 and whose outputis connected to the control valve 19, to an independently derived value,i.e., the set value of controller 43, with this type of control, if theliquid level in the bottom of the column changes due to a change in theliquid flow in the stripping section caused by adjustment of the valveto maintain the quality of the bottom product, it is not possible tocorrect the column bottom liquid level in the usual manner via bottomproduct flow line 18, since the bottim product flow is maintained at aconstant value. Accordingly, control of the liquid level in the bottomof the column is provided by passing the output signal from levelcontroller 20 to the set value of input of heat flow controller 22.Controller 22 adjusts the liquid level in the bottom of the column '1 toits desired value by increasing the heat flow to the reboiler upon anincrease in the liquid level in the bottom of column 1 and vice versa. Achange of the liquid level in the bottom of the column is naturallyaccompanied by a change of the vapor flow through the entire column. Anychange in the vapor flow in the rectifying section of the column is,however, immediately compensated for by the ratio controller 33. As withthe embodiment of FIGURE 1, with this variant of the invention thetemperature at which the feed stream enters the column is of noconsequence. The set value for controller 33 may, if desired, be derivedfrom a meter measuring the quality of the top product (not shown).

Furthermore, for the processes whereby the top product flow or thereflux flow is maintained at an independent value and in relation towhich embodiments are given (FIGURES 3-5) which are tied to therequirement that the feed stream is introduced into the column at atemperature equal or approximately equal to the boiling temperature,within the range of the invention embodiments can be presented which aresuitable for feed streams that enter the column at a temperature belowthe boiling temperature. In these embodiments the heat flow to the feedpreheater is then adjusted, by a signal derived from a flowmetermeasuring the feed stream and from the difference in temperature betweenthe feed stream leaving the feed preheater and the liquid on the feedtray, in such a way that the contribution from the feed stream to thevapor flow in the rectifying section of the column is kept at a desiredvalue. An example of this modification of an embodiment of a controlscheme according to the invention wherein the top product flow isadjusted at an independent value, i.e., the control scheme of FIGURE 3,is given in FIGURE 7.

According to this modification, a meter 5'0 is provided which measuresthe difference in temperature between the feed stream leaving the feedpreheater 3 and the liquid on the feed tray in the column 1. The outputsignal from the meter 50 and from a flowmeter 51 for measuring the feedflow rate are fed to respective inputs of a multiplier 52 which producesan output signal proportional to the product of the two input signalsand an adjustable constant. The output from multiplier 52 is fed to thenormal input of a controller 53 whose output controls the valve 54 inthe flow line 55 for the supply of heating medium to the feed preheater3. The controller 53 varies the position of the valve 54 to maintain aconstant output from multiplier 52 and thereby maintain the contributionfrom the feed stream to the vapor flow in the rectifying section of thecolumn at a desired value. In the absence of this additional control, ifthe temperature of the feed stream is below the boiling temperature,then a portion of the vapor flow in the rectifying section of the columnwill be condsensed by the feed stream. Accordingly, any change in thefeed stream resulting from a control action according to the inventionwill cause the vapor flow in the rectifying section to change as well.This change of the vapor flow in the column is prevented by increasingthe heat flow to the preheater 3 to a suflicient extent. A similareffect can be obtained with a feed stream above the boiling temperature,if the heat supply to the preheater is replaced by a heat dischargemedium.

It should be noted that the combination of the control of the heat flowto the feed preheater with the control according to the inventionwhereby the reflux flow is maintained at an independent value (seeFIGURE 5) is not represented by a figure. However, it is understood thatthe control system formed by units 50-55 can equally well be applied tothis embodiment of the invention.

Naturally, the two last-mentioned modes of control, whereby the heatflow to the feed preheater is adjusted, can also be used if thetemperature of the feed stream is equal or approximately equal to theboiling temperature on the feed tray.

In the aforementioned examples the manner in which the flow of coolingmedium through the condenser 6 is adjusted is not indicated. However,systems for controlling the degree of cooling by the condenser are oldand well known in the art and take many forms depending on the desiredmode of control. For example, known systems are available for adjustingthe cooling medium flow to control the pressure in the column or tooptimize the heat economy of the process. This also applies to theadjustment of the heat flow to the feed preheater in those cases whereadjustment of that heat flow is not used. These additional controlsystems can be combined with the process according to the invention.

It is not necessary for the quality meters to be connected directly tothe top product flow or the bottom product flow. Sometimes it may bedesirable, for example, in view of the accuracy of the quality meter oranalyzer, to connect this meter to the liquid on a tray in the column.The control systems mentioned hereinbefore may be pneumatic, hydraulic,electric, electronic or mechanical systems or may constitute a mixtureof these types.

We claim as our invention: 1. In a process for the continuousdistillation of an input feed stream into top and bottom productsstreams in a multi-stage distillation column in which overhead vaporsare condensed in a top product accumulator and both reflux in the upperportion and re-evaporation in the lower portion of the column areapplied, wherein the process variables include a feed stream flow rate,a degree of preheating of the feed stream, a reflux flow rate, a topproduct stream flow rate, a bottom product stream flow rate, a degree ofcooling in the condenser, and a quantity of heat supplied to thereboiler for re-evaporation; a control method comprising:

continuously analyzing the quality of the bottom product to determineany deviation between the measured and desired values of quality of saidbottom product;

adjusting the feed stream flow rate to maintain the quality of thebottom product at the desired value;

adjusting the reflux flow and the top product flow to maintain theliquid level in the top product accumulator within desired limits andthe ratio between the top product flow and the reflux flow at aspecified value;

adjusting the bottom product stream flow to maintain the liquid level inthe bottom within specified limits; and

maintaining the quantity of heat to the reboiler at a constant value.

2. The method of claim 1 wherein the feed stream is introducted into thecolumn at a temperature below the boiling temperature of said feedstream and including the steps of:

maintaining the reflux flow at a constant value;

adjusting the top product flow to maintain the liquid level in theaccumulator between specified limits;

adjusting the bottom product flow to maintain the liquid level in thebottom of the column between specified limits;

maintaining the heat flow to the reboiler at a fixed value;

measuring the flow rate of the feed stream;

detecting the difference between the temperature of the feed streamleaving the feed preheater and the liquid on the column feed tray; and

utilizing the measured value of the feed flow and the value of saidtemperature diflerence to adjust the heat flow to the feed preheater tomaintain the contribution from the feed stream to the vapor flow in therectifying section of the column at a predetermined value.

3. The method of claim 2 wherein the top product flow is maintained at aconstant value and the reflux flow is adjusted to maintain the liquidlevel in the accumulator between desired limits. 4. The method of claim2 wherein the step of utilizing the value of the feed stream flow andthe temperature difference to adjust the flow of heat to the feedpreheater comprises:

multiplying a signal corresponding to the measured value of the feedflow by a signal corresponding to 5 the value of the said temperaturedifference and by a predetermined constant; passing the resultingproduct signal to the normal input of a controller; and adjusting theheat flow to the feed preheater by means of the output signal from saidcontroller to maintain said product signal at a predetermined value.

5. The method of claim 3 wherein the step of utilizing the value of thefeed stream flow and the temperature difference to adjust the flow ofheat to the feed preheater comprises:

multiplying a signal corresponding to the measured value of the feedflow by a signal corresponding to the value of the said temperaturedifference and by a WILBUR L. BASCOMB, 111., Primary Examiner.

US. Cl. X.R.

